Spoiler: the first part of this post is largely inspired by this very nice paper written by Neville Blampied, as well as by this great blogpost from Matti Vuorre.

Nomothetic and idiographic research

We can distinguish between (at least) two research traditions in psychology. The first one is called nomothetic research, and is defined as research intended to discover general laws at the population level. This is arguably the most common form of investigation in cognitive, social or clinical psychology.

Yet, over the years, some authors have consistenly argued that determining experimental effects or therapy efficiency by only relying on group means (i.e., by aggregating outcomes) do not neccesarily apply to any specific individual, because the group mean may conceal individual nonresponse (Blampied, 2017). Some authors also pointed out that the hypothesised mean individual do not usually refers to any actual individual being (e.g., Vautier, 2011, 2014). As an alternative, the idiographic approach rather considers individuals as the unit of analysis (e.g., Lamiell, 1998; Barlow & Nocke, 2009).

These critics lead to alternative ways of analysing, reporting, and displaying data (say goodbye to bar graphs). In line with the idiographic approach, we will discuss a type of graphical representation in which individuals are the primary unit of analysis. This kind of visualisation is especially well suited for studying changes over time, and allows simultaneously displaying both idiographic (i.e., individual changes) and nomothetic (i.e., mean group effects) information.

The modified Brinley plot

The first version of this plot has been suggested by Brinley (1965), who plotted group means of two groups on different tasks, one against the other on the same plot (the first one on the x-axis and the second on the y-axis, see the first figure of this post). When there is no differences between groups, individual data points are aligned on the 45° line. Points above this line represent observations that are higher on the y-axis score than on the x-axis score (and reversely for points below the 45° line).

This plot has been used quite widely in the subsequent decades (usually without referring to the original idea of Brinley), and has been modified to display individual data points (rather than group means), allowing for instance to display within-subject effects from a pre-manipulation to a post-manipulation score.

After reading Blampied’s paper, I managed to write down a short function for making this kind of plot easily. You can find this function in the lmisc package (available on Github).

A first basic Brinley plot can be obtained by using the brinley function, specifying the formula as you would do using lm or plot1. The black cross represents the bivariate mean and the confidence interval around the mean in the two conditions.

brinley(brinley_data, outcome ~ session)

There are multiple ways to compute a Cohen’s d effect size for within-subject designs (e.g., see this post for five different ways to do it). By default, the brinley function computes the average Cohen’s d (as recommendended by Cumming, 2012), but note that this can easily be modified to compute your favourite Cohen’s d.

Next (here comes the augmented modified Brinley plot), we can colour points by a second continuous variable, to be passed (as character) to the colour argument.

brinley(brinley_data, outcome ~ session, colour = "pain")

Or facetting by another categorical variable, using the facet argument, which is quite convenient to compare the effects of a pre-post manipulation in different groups/experiments.

brinley(brinley_data, outcome ~ session, facet = "condition")

Moreover, the brinley function uses a smart plotting trick that allows easy comparison of multiple groups by plotting the whole dataset in background (see this blogpost). In our case (two groups), the grey dots represent data from the other group2.

Illustrative example

Let’s say we gathered data from two experiments (exp1 and exp2) in which we investigated a within-subject effect, in two conditions group1 and group2.

This plot reveals two overlapping distributions of scores in each experiment. Visually, it seems that the difference in means (indicated by the vertical dashed lines) is a bit lower in exp2 than in exp1. However, the standard deviation (the width) of the distributions in exp2 seems also a bit lower than in exp1.

Intuitively then, although the absolute mean difference seems lower in exp2 than in exp1, we could say that the effect size (defined as the standardised mean difference) might be roughly equivalent across the two experiments. Let’s check this.

And indeed it is. We have the exact same effect size in the two experiments3. But does it mean that the effect is the same in the two experiments ? Well, this is an very ill-defined question, but another way to compare effects (beyond merely comparing the d value) would be to ask how much variable is the considered effect across participants…

Stochastic dominance

This relates to the idea of dominance constraint or dominant effect (Rouder & Haaf, 2018), which is the idea according to which there exists some effects that are (in the population) in the same direction for any individual, although this might be masked in empirical observations because of sampling or environmental noise.

How is this relevant here ? Well, although we presented data from two experiments with equal sample size and equal effect size, maybe the idea of dominance could help us to reach a more fine-grained interpretation of this effect. Below, we use a modified Brinley plot to visualise data issued from these two experiments.

brinley(data = dominance, formula = value ~ group, facet = "exp")

This plot reveals that, altough the mean effect size is the same accross the two experiments, the effect observed in exp2 seems prototypical of a dominant effect, that is, all participants (all points) show an increase from group1 to group2. In contrast, the effect observed in exp1 seems less “universal”, with some participants showing the reverse effect.

Of course, this is a schematic representation of an idealised dominant effect, in the sense that it would correspond to a situation in which there is absolutely no sampling noise. Alternatively, we could also consider that exp2 is a representation of a dominant effect in the population (i.e., a representation of what a dominant effect would look like if there was no sampling noise), while data from exp1 would be a representation of a dominant effect in ecological/realistic experimental settings (i.e, with some moderate sampling noise).

In any case, this highlights the need to report and to visualise data at different levels (individual vs. group) and in different ways to have a better grasp on what is happening in a particular experiment. In this perspective, modified Brinley plot appears as a handy tool to explore the idea of dominance constraint, and more generally, to assess to what extent our experimental manipulations affect the participants.